One type of pump assembly used particularly in oil producing wells has a submersible pump driven electrical motor filled with a dielectric motor lubricant. The motor rotates a shaft assembly to drive the pump. A seal section connects between the motor and the pump. The seal section has a pressure equalizing element that reduces a pressure differential between lubricant into the motor and well fluid on the exterior. The pressure equalizing element is typically an elastomeric, flexible bag or a metal bellows. Motor lubricant in communication with motor lubricant in the motor fills the interior of the pressure equalizing element. A well fluid communication port admits well fluid into the seal section on the exterior of the pressure equalizing element.
A shaft seal, which is normally a mechanical face seal, seals well fluid from entry into the pressure equalizing element. The shaft seal includes a rotating element or runner that rotates with the shaft. An elastomeric boot and spring urge the seal runner against a stationary base. Slight leakage occurs at the interface between the seal runner and seal base for lubrication.
The pressure equalizing element flexes to equalize the lubricant pressure in the bag with well fluid pressure on the exterior of the seal section. If the pressure differential at the interface between the seal runner and the seal base is equal to or nearly zero, there is no control on the direction of leakage at the interface between the seal runner and seal base. A zero pressure differential not only allows the well fluid to leak inside the pressure equalizing element, it can also cause overheating between the seal runner and seal base due to the lack of lubrication and cooling. Generally, mechanical face seals ran more stable and last longer when there is a small amount of differential pressure at the interface.
Prior art seal sections may have a check valve that allows some of the lubricant in the pressure equalizing element to expel due to thermal expansion of the lubricant. However, the check valve normally retains a differential pressure of the lubricant pressure above the well fluid pressure only when the lubricant in the pressure equalizing element is at a maximum expansion. Daring operation over a long period of time, the lubricant will typically diminish in volume.